Many image projection apparatuses which project images using a laser beam have been proposed. For example, a laser scan display has been disclosed in Japanese Patent Laid-Open No. 2003-21800, which displays images on a screen by scanning the laser beam with a scanning device.
However, since the laser beam has high coherence, a speckle noise (that is, a granular interference pattern) which causes degradation of image quality appears in the displayed image.
A method for reducing a speckle noise has been disclosed in Japanese Patent Laid-Open No. 2000-206449, in which a transparent optical element having a refraction index n and including “N” areas whose thicknesses change by “Δt” is used. In the method, laser light that is a diverging luminous flux from a semiconductor laser (laser diode) is converted into a parallel luminous flux by a collimator lens, and enters the transparent optical element. The transparent optical element gives optical path differences “(n−1)Δt” to luminous fluxes (divided luminous flux) passing through portions with different thicknesses, thereby reducing the coherence of each divided luminous flux. It is possible to reduce the speckle noise by overlapping the incoherent luminous fluxes that have passed through the transparent optical element by a lens.
Furthermore, a method for reducing a speckle noise has been disclosed in Japanese Patent Laid-Open No. H06-208089, in which a laser beam is scattered with a rotatable diffusing element. The speckle pattern is changed at a speed undetectable to the human eye by rotating the diffusing element. Overlapping the speckle pattern changing at a high speed by the eye's afterimage effect makes it possible to reduce the speckle noise.
In addition, as an art for smoothing a speckle distribution to equalize the intensity distribution of a laser beam emitted and condensed in a laser-beam-condensing system, a method has been disclosed in Japanese Patent Laid-Open No. H10-39267, in which a polarized beam is divided into two luminous fluxes having mutually-orthogonal polarization directions by using a birefringent crystal or the like.
However, in the method disclosed in Japanese Patent Laid-Open No. 2000-206449, it is necessary to increase the step “Δt” of the transparent optical element to give enough optical path differences to the divided luminous fluxes. As a result, it becomes difficult to downsize the optical system. In particular, the size of the transparent optical element becomes larger when using light having a long coherence length such as a solid-state laser and a single-mode semiconductor laser.
Furthermore, in the method disclosed in Japanese Patent Laid-Open No. H06-208089, a loss of light amount becomes relatively large because the laser beam is transmitted through the diffusing element.
Furthermore, since the art disclosed in Japanese Patent Laid-Open No. H10-39267 is premised on a laser system used in nuclear fusion apparatuses, it is difficult to apply the art to displaying optical systems and image projection apparatuses.